The present invention relates to components and methods for mounting and connecting microelectronic elements such as semiconductor chips.
Complex microelectronic devices such as semiconductor chips require numerous connections with other electronic components. Typically such semiconductor chips are mounted on external substrates such as printed circuit boards by electrically interconnecting contacts on the semiconductor chip with contact pads on the substrate. The substrate may include internal circuitry which is connected to the contact pads thereof and may be adapted to accommodate other components such as additional semiconductor chips.
Connections between microelectronic elements and substrates must meet several demanding and often conflicting requirements. The connections must provide reliable low impedance electrical interconnections. They must also withstand stresses occurring during manufacturing processes, such as the thermal effects caused by soldering. Other thermal effects occur during operation of the device. As the system operates, it generates heat which causes the chip and the substrate to expand.
When operation ceases, the chip and the substrate cool down which causes the components to shrink or contract. As a result, the chip and the substrate expand and contract at different rates so that portions of the chip and substrate move relative to one another. The chip and the substrate can also warp as they are heated and cooled, thereby causing further movement of the chip relative to the substrate. The repeated expansion and contraction of the elements during operation results in severe strain on electrical elements connecting the chip and the substrate.
In response to these problems, various interconnection systems have evolved. These systems essentially seek to withstand repeated thermal cycling without breaking the electrical connections. The interconnection system should also provide a compact assembly and should be suitable for use with components having closely spaced contacts. Moreover the interconnection system should be economical. Various solutions have been proposed to meet these needs. Some embodiments of commonly assigned U.S. Pat. Nos. 5,148,265 and 5,148,266 teach that flexible leads may be provided between the contacts on a semiconductor chip or other microelectronic element and contact pads of a substrate. Preferably a compliant layer, such as an elastomer or gel, may be provided between the semiconductor chip and the substrate, whereby the flexible leads connecting the semiconductor chip and substrate extend through the compliant layer. In certain preferred embodiments, the semiconductor chip is mechanically decoupled from the substrate so that the chip and the substrate can expand and move independently of one another without placing excessive stress on the electrical connections (i.e. the flexible leads) between the chip contacts and the contact pads of the substrate. The chip and the flexible leads extending to the substrate typically occupy an area of the substrate about the same size as the chip itself.
Commonly assigned U.S. patent application Ser. No. 08/641,698, the disclosure of which is hereby incorporated by reference herein, discloses connection components for microelectronic assemblies. In accordance with certain preferred embodiments of the '698 application, the microelectronic assemblies preferably include first and second microelectronic elements having contacts thereon and a compliant dielectric material having cavities therein. Masses of a conductive material are disposed in the cavities so that the masses of the conductive material are electrically interconnected between contacts on the first microelectronic element and contacts on the second microelectronic element. Thus, each conductive mass forms part or all of a conductor extending between contacts on the two microelectronic elements. The conductive material may be a liquid or may be a fusible material adapted to liquefy at a relatively low temperature, typically below about 125.degree. C. Preferably the conductive material in each mass is contiguous with the compliant material and is contained by the compliant material so that the conductive material remains in place when in a liquid state. The compliant material keeps the liquid masses associated with different sets of contacts separate from one another and electrically insulates the masses from one another.
Commonly assigned U.S. patent application Ser. No. 08/962,693 entitled "Microelectronic Connections with Liquid Conductive Elements," filed Nov. 3, 1997 (as a CIP of the above-mentioned '698 application), the disclosure of which is hereby incorporated by reference herein, discloses microelectronic assemblies having conductive elements which transfer heat between microelectronic elements. In certain embodiments, first and second microelectronic elements are juxtaposed with one another so that the confronting spaced apart surfaces of the first and second microelectronic elements define a space therebetween. One or more masses of a conductive material having a melting temperature below about 150.degree. C. (hereinafter referred to as "fusible conductive material") are provided in the space. The fusible conductive material is preferably thermally conductive, electrically conductive or both. A compliant layer is provided in the space between the microelectronic elements. The fusible conductive masses are preferably contained by the compliant layer and may extend between contacts on the confronting surfaces of the first and second microelectronic elements so that the masses electrically interconnect the first and second microelectronic elements. The fusible conductive masses also preferably provide a thermal conduction path between the first and second microelectronic elements.
Commonly assigned U.S. patent application Ser. No. 08/862,151 filed May 22, 1997, the disclosure of which is hereby incorporated by reference herein, discloses connectors for microelectronic elements. In certain preferred embodiments of the '151 application, a microelectronic element is engaged with a connecting assembly. The connecting assembly preferably includes a flexible dielectric interposer, a substrate and non-collapsible structural elements which support the flexible dielectric interposer above the substrate, leaving a standoff space between the dielectric interposer and the substrate. In one preferred embodiment a microelectronic element having a plurality of contacts protruding from the bottom surface thereof is engaged with the flexible dielectric interposer. The contact bearing surface of the microelectronic element is juxtaposed with the top surface of the flexible dielectric interposer whereby the contacts of the microelectronic element are generally in registration with an array of contacts provided on the flexible dielectric sheet. The microelectronic element is then urged downwards so that contacts engage the contact pads of the flexible dielectric sheet. Downward motion of the microelectronic element relative to the flexible dielectric sheet resiliently deforms the flexible sheet with each microelectronic element contact resiliently deflecting a surrounding portion of the flexible sheet downward into a standoff space between the flexible sheet and the substrate. As the surrounding portion of the sheet-like element is deflected into the standoff space, the flexible sheet is stretched. The structural elements overlying the substrate force the flexible sheet-like element upward as the microelectronic element contacts force the sheet-like element downward.